Biomacromolecules 2005, 6, 1055-1061
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Elastic Modulus and Stress-Transfer Properties of Tunicate Cellulose Whiskers Adriana S ˇ turcova´ ,† Geoffrey R. Davies,‡ and Stephen J. Eichhorn*,† Materials Science Centre, School of Materials, University of Manchester, Grosvenor Street, Manchester, M1 7HS United Kingdom, and IRC in Polymer Science and Technology, School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT United Kingdom Received November 8, 2004; Revised Manuscript Received December 21, 2004
Experimental deformation micromechanics of natural cellulose fibers using Raman spectroscopy and X-ray diffraction have been widely reported. However, little has been published on the direct measurements of the mechanical properties, and in particular the elastic modulus, of the highly crystalline material in the native state. Here we report on measurements of the elastic modulus of tunicate cellulose using a Raman spectroscopic technique. A dispersed sample of the material is deformed using a four-point bending test, and a shift in a characteristic Raman band (located at 1095 cm-1) is used as an indication of the stress in the material. Relatively little intensity change of the Raman band located at 1095 cm-1 is shown to occur for samples oriented parallel and perpendicular to the polarization direction of the laser, as compared to a highly oriented flax sample. This indicates that the tunicate sample is a two-dimensional in-plane random network of fibers. By use of this result, the Raman shift, and calibrations with strain from other materials, it is shown that the modulus of the material is very high, at about 143 GPa, and a lack of Raman band broadening is thought to be due to the fact that there is pure crystalline deformation occurring without the effect of crystalline/amorphous fractions. A strain sensitivity of the shift in the 1095-cm-1 Raman peak for this specimen is shown to be -2.4 ( 0.2 cm-1/%. A molecular mechanics approach, using computer simulation and an empirical force field, was used to predict the modulus of a highly oriented chain of the material, and this is found to be 145 GPa, which is in agreement with the experimental data. However, by use of a normal-mode analysis, it is found that a number of modes have positions close to the central positions of the experimental Raman band. One in particular is found to shift at a rate of 2.5 cm-1/%, but due to the complex nature of the structure, it is not entirely conclusive that this band is representative of the experimental findings. Introduction Tunicin, a cellulosic material produced by a marine animal, has been shown1 to consist predominantly of the Iβ allomorph. Tunicate cellulose, due to its high crystallinity, has also been used as a model structure to determine the crystal and molecular structure and hydrogen-bonding system in cellulose Iβ.2 Cellulose whiskers produced by tunicates have a large aspect ratio,3 having lengths between 1160 and 2000 nm with cross-section diameters of about 15 nm. They are also known4,5 to have a high specific surface area in the range ∼150-170 m2/g, a high crystallinity4 of 95%, a reactive surface due to hydroxyl groups, and are thought to have high mechanical properties, making them suitable candidates for composite reinforcement. Two-phase composite materials can be prepared where a less oriented polymer forms the soft phase and dispersed fibers or whiskers form the stiff phase.6 Tunicate whiskers * To whom correspondence should be addressed. E-mail: s.j.eichhorn@ manchester.ac.uk. Telephone: 0161 200 5982. Fax: 0161 200 3636. † University of Manchester. ‡ University of Leeds.
have been used in such materials, with soft-phase materials such as starch5 or waterborne epoxy Reapox 164, a diglycidyl ether of bisphenol-A prepolymer.4 In the case of the waterborne epoxy Reapox 164, a significant improvement in dynamic mechanical properties was observed even with a low concentration of cellulose whiskers.4 The reinforcement is thought to be a result of a hydrogen-bond network formed between the cellulose whiskers. In addition there are strong interactions between hydroxyl groups at the cellulose surface, and polar sites of the polymer chains that reduce molecular mobility of the epoxy chains close to the surface of the whiskers.4 On the other hand, the addition of tunicin whiskers to glycerol-plasticized starch matrixes resulted only in a relatively low reinforcement.5 It is possible that competitive interactions between the components and plasticizer accumulation at the cellulose/amylopectin interface interferes with the hydrogen-bonding forces between whiskers, therefore hindering the stress-transfer at the filler/matrix interface.5 This behavior differs from that reported for glycerolplasticized starch filled with cellulose microfibrils in which a reinforcing effect most likely occurs due to the formation of a hydrogen-bonded cellulose network within the matrix and due to tangling of the flexible cellulose microfibrils.5
10.1021/bm049291k CCC: $30.25 © 2005 American Chemical Society Published on Web 02/15/2005
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The elastic modulus of the crystalline region of cellulose is an important property of the polymer, especially with respect to the ultimate aim of exploiting its full potential in composite materials. Several attempts have been made to determine the value of the elastic modulus of cellulose either experimentally or theoretically.The first modeling of cellulose mechanical properties by Meyer and Lotmar7 in 1936 was later corrected by Lyons.8 Further modifications were made to this calculation by Treloar.9 However, experimentally the crystal deformation of cellulose I using highly oriented fibers of bleached ramie was first studied by Sakurada et al.10 in 1962 and separately, in the same year, by Mann and RoldanGonzalez.11 Sakurada et al.10 obtained a value of 137 GPa for cellulose I. However, Mann and Roldan-Gonzalez’s values11 were in the range 70-90 GPa for the elastic moduli of the cellulose I and cellulose II crystals, respectively. These values were determined from the observation of a change in the c spacing measured by X-ray diffraction on deformed fiber bundles. Matsuo et al.12 found the crystal lattice modulus of cellulose I was in the range 120-135 GPa, when similar X-ray diffraction experiments were carried out on ramie cellulose. These values were in agreement with the value obtained by Sakurada et al.10 and with a value of 136 ( 6 GPa calculated by Kroon-Batenburg et al.13 for the Young’s modulus along the chain axis at room temperature. However, these values differed from the theoretical estimate of 167.5 GPa reported by Tashiro and Kobayashi.14 The latter value is thought to be higher due to the fact that their calculation was carried out at low temperature, where the thermal motion of the chain was “frozen”. Matsuo et al.12 carried out a detailed analysis of the relationship between the measured values of the elastic modulus and the molecular orientation and crystallinity. They concluded that values of the experimental and theoretical crystal modulus are almost identical when a series coupling between amorphous and crystalline phases is predominant, but they are different when a parallel coupling is present. However, they found that increasing the degree of molecular orientation and crystallinity decreased the significance of this morphological dependence. They recommended the use of films and/or fibers with a high degree of molecular orientation, and high crystallinity, for the determination of the crystal lattice modulus by X-ray diffraction. However, it is difficult to perceive how an accurate determination of the crystal modulus of highly crystalline cellulose samples could be carried out since the specimens would be difficult to grip, etc. Mitra et al.15 were the first to observe a shift in Raman band positions under the application of a tensile stress to a monocrystalline polydiacetylene fiber. They and others16 have shown that the shift in Raman band position with deformation is caused by an anharmonicity of the bond. However, the first characterization of unstrained cellulosic material by Raman spectroscopy was carried out by Blackwell et al. in 1970 on highly crystalline cellulose obtained from the algae Valonia Ventricosa.17 Atalla18 furthered the understanding of cellulose polymorphism by the use of the same technique, and a Raman spectroscopic study by Wiley and Atalla19 provided more thorough characterization of the Raman bands in cellulose.
Micromechanical studies similar to those conducted by Mitra et al.15 have been carried out on various forms of cellulose.20-22 The forms of cellulose characterized involved natural (flax and hemp) and regenerated fibers, microcrystalline cellulose, and natural composites such as paper and wood. The rate of Raman band shift was observed to be invariant with stress for all of the fiber types studied, which indicates that the fiber microstructure is adequately described by a modified-series aggregate model.20-22 The modifiedseries aggregate model predicts that the rate of the Raman band shift with strain is proportional to the fiber modulus, E, and this has been confirmed experimentally.21,22 However, in this model, the stress is assumed to be uniform, and equal in both crystalline and amorphous regions, which excludes any broadening effects on the Raman band caused by distribution of stresses over the individual components of the microstructure. This contradicts recent observations23 made on high performance fibers derived from liquid crystals. In that study, Raman bands broadened under stress and strain in low-modulus fibers but not in high-modulus fibers. The authors suggested that a hybrid model structure with parallelseries arrangement of amorphous and crystalline domains (first suggested by Takayanagi et al.24) represents more closely the morphology of these fibers. They also used a modeling technique to predict the modulus of cellulose II, which was found to be 98 GPa. The very high performance mechanical properties, apart from other factors, enable tunicin to achieve the desired level of reinforcement in some composite materials.4 However, there are no reports of direct measurements of the modulus of single whiskers of cellulose such as tunicin. Their high crystallinity makes tunicin, according to the findings of other workers,12 a suitable material for determination of the crystal modulus. Dispersion of the tunicin material in an epoxy resin enabled us to deform it and monitor the shift in the position of the structurally significant 1095-cm-1 Raman band with applied external deformation. The observed rate of the Raman band shift with strain, and previously observed relationships between band shift rate and modulus, were used to determine the elastic modulus of tunicate cellulose for the first time and make conclusions about the mode of stress transfer. We also performed calculations on a cellulose model under tension and confirmed the experimental predictions of chain stiffness. Experimental Methods Materials. Cellulose microcrystals (whiskers) were extracted from the mantle of a tunicate sea animal. The mantle is formed from well organized and highly crystalline cellulosic fibrils. The tunic of the animals was cut into small fragments, which were deproteinized by three successive bleaching treatments using hydrogen peroxide and a previously published method.25 The bleached mantles were disintegrated in water, first by using a Waring blender at a concentration of 5 wt % and then via 15 passes through a Gaulin laboratory homogenizer operated at 400 bar at a concentration of 1 wt %. The aqueous tunicin suspension obtained was mixed with H2SO4. When a final acid/water
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and a new spectrum was obtained. This process was repeated up to a maximum strain value of 1.5%. In total, four samples were tested using this method. Each set of spectra from these experiments was fitted using a mixed Gaussian/Lorentzian function, using the GRAMS-32 software, to determine the peak positions as a function of strain. The laser in the Raman spectrometer was polarized parallel to the tensile direction of the surface of the resin beams, and the main axis of the flax fiber samples for the orientation studies. Theoretical Methods Figure 1. Schematic diagram of the 4-point bending test used to deform the epoxy/tunicate composites.
concentration of 55% weight fraction was reached, the tunicin was hydrolyzed at 60 °C for 20 min under intense stirring. Thus, a dispersion of cellulose whiskers was obtained, which was then sonicated, neutralized, and washed by dialysis. A “paperlike” sheet of cellulose whiskers was then subsequently formed by solvent extraction and deposition. Samples of steam-exploded flax were also used for the orientation analysis. These fibers were used as provided, with no surface treatments (which have been applied before21,22) before testing. Preparation of Composite Samples. Epoxy resin beams with approximate dimensions of 60 × 10 × 3 mm3 were prepared from a mixture of equal amounts of Araldite epoxy resin LY5052 and hardener HY5052 (from Sigma Aldrich). A rectangular piece of a tunicin sheet was secured to the surface of the pre-prepared resin beam using the same resin mixture. During a seven-day cold curing process, the epoxy resin/hardener mixture was allowed to penetrate into the porous sheet of the tunicate cellulose, resulting in the formation of a tunicin/epoxy composite. Finally, a miniature strain gauge type EA-06-240LZ-120 (Measurements Group, Inc., North Carolina, USA) was attached to the beam in close proximity to the cellulose sheet with cyanoacrylate adhesive. Single fibers of steam-exploded flax were also mounted onto a cardboard window using cellotape, with the main axis of fibers being parallel to the longer edge. Deformation of Composite Samples. The epoxy beams were deformed using a 4-point bending mode test as has been described before for cellulosic samples20 under the microscope of a Renishaw System-1000 spectrometer. A pictorial representation of the deformation of the samples is shown in Figure 1. Raman spectra were recorded using a 632-nm helium-neon laser for tunicin and a 785-nm laser for flax, focused to a spot of 2 µm using a ×50 lens (numerical aperture ) 0.60). The full power of the lasers was 30 and 26 mW for the 632- and 785-nm systems, respectively. However, the laser power on the sample is about 1-2 mW. After the laser was focused onto a selected area of the tunicin/epoxy composite, a spectrum was obtained. The exposure time for a single scan was 10 s, and since the number of scans accumulated in a single spectrum was 30, the total time for each spectrum was 300 s. After the spectrum was acquired, the strain was increased by 0.05%,
Software capable of molecular mechanics simulation, namely, Accelrys Materials Studio Version 2.2 with the Discover minimization package on a desktop PC with a COMPASS force field, was used to deform a theoretical cellulose I structure. The initial atomic coordinates for this structure were taken from data published by Cael et al.26 Since we did not wish to achieve a model structure for fully crystalline material, as the normal-mode analysis and interchain interactions are complicated, this simple model, as opposed to a full Iβ model of the atomic coordinates,2 was used. A single chain of cellulose was placed in a periodic cell, with a and b spacings of 20 Å to prevent any interactions between the chains. Such interactions, as previously described,23 could potentially increase the stiffness of the structure through a reduction in the lateral deformation. However, this approach was chosen to simplify the analysis, but we should expect that the chain modulus will be higher than for an experimental structure, since intrachain hydrogen bonding will be maximized. This initialization of the structure was followed by all atom energy minimizations of the system using a series of different c spacings while fixing the cell dimensions a and b, which yielded the potential energy as a function of chain length. A normal-mode analysis was performed at each strain level during this procedure. The software can only predict infrared intensities; therefore, a selection procedure had to be employed. Bands of low infrared intensity close to the experimentally determined position of the 1095-cm-1 band were selected. This was done since it is usually the case that infrared-active vibrational modes are less likely to be Raman active.27 Results and Discussion Raman Spectroscopy. Clear Raman spectra can be obtained from tunicate cellulose, even when it is embedded in the epoxy resin, and a typical example is presented in Figure 2, where the peak at about 1095 cm-1 is highlighted. This band corresponds to the vibration of bonds within the backbone of the cellulose molecule and is dominated by the C-O stretching motion, which is almost parallel to the chain axis. In this study, only the relatively narrow region of the Raman spectrum of cellulose, between 1000 and 1150 cm-1, has been investigated. As already mentioned, bands in this region correspond to the motions of the atoms in the molecular backbone of cellulose, and will be the most sensitive to the effects of chain deformation. Figure 3 reports
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Figure 2. A typical Raman spectrum for a tunicate sample highlighting the position of the 1095-cm-1 band.
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Figure 4. A typical Raman band shift of the 1095-cm-1 peak of tunicate cellulose subjected to tensile strain.
Figure 5. The Raman band shift in the 1095-cm-1 as a function of strain for four independent experiments on tunicate cellulose.
Figure 3. Typical changes in the intensity of the 1095-cm-1 Raman band for (a) a flax fiber and (b) tunicate samples when oriented parallel and transverse to the laser polarization direction.
the effect on the intensity of the 1095-cm-1 band when the tunicate sample was rotated by 90° to the polarization direction, compared to a similar rotation of a flax fiber sample. It can be seen that there is a significant decrease in the 1095-cm-1 peak intensity when the flax fiber orientation is rotated from a parallel to perpendicular position relative to the polarization direction (see Figure 3a). It is thought that this large variation in the C-O ring stretching peak intensity is a direct result of the highly oriented cellulose chains within the flax cell wall, as has been noted before for wood tissue.28 The majority of the cellulose microfibrils in flax fiber cell wall have also been found to be orientated parallel to the longitudinal fiber cell axis.29-31 In contrast, there is little relative change in the intensity of the 1095cm-1 peak when the alignment of the tunicate sample is rotated from a parallel to perpendicular position relative to the polarization position. This observation is consistent with a random orientation of the tunicin whiskers in a two-
dimensional plane of the sheet and is a result that will be used later to determine the crystal modulus of a single tunicate filament. When the tunicin/epoxy composites were subjected to deformation in a 4-point bending test, as previously described, the 1095-cm-1 peak shifted toward lower wavenumbers. A typical shift of this band with strain is shown in Figure 4. This behavior indicates that stress transfer is taking place in the composite between the epoxy matrix and the tunicin whiskers, resulting in direct stressing/straining of the molecular backbone of cellulose.20-23 The exact position of the peak at 1095 cm-1 in the tunicin/epoxy composite is linearly dependent on strain up to a value of about 0.8% but reaches a plateau at higher values (cf. Figure 5). The gradient, d(∆ν)/d, of the initial linear shift in the composite was found to be -2.4 ( 0.2 cm-1/%. This is a best-fit value, determined by fitting the data points collected within the linear region of four tunicin/epoxy composite samples. The magnitude of the wavenumber shift with strain in a material depends on the chemical structure, its morphology, and its microstructure. Indeed, celluloses from various sources20-23 gave varying values of the rate of the Raman band shift with strain (d(∆ν)/ d). However, the tunicate sample is thought to be highly crystalline and therefore relatively free from the influence of amorphous material. This is confirmed by the fact that
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no broadening of the composite 1095-cm-1 peak with strain is observed (cf. Figure 4), which indicates that there is no distribution of stress and strain over amorphous/crystalline domains. A recent study23 has shown that Raman band shifts of the 1095-cm-1 peak in cellulose II fibers depends on the relative fractions of amorphous and crystalline material and how they are arranged in the structure. It was also shown that the higher modulus fibers, and therefore usually the more crystalline the sample, the lower the Raman band peak broadening with stress.23 Therefore, we conclude that the shift obtained for tunicin indicates pure chain deformation of crystalline cellulose material. The presence of the plateau in the curve describing the dependence of the Raman band shift with strain (cf. Figure 5) could indicate a decreased efficiency of the stress transfer between the epoxy matrix and the tunicin whiskers. This may be due to a weakening or a breakdown of the whisker-matrix interface, as has been seen with nanocomposite materials using the same method.32 It has been previously shown that the band shift rate with stress (d(∆ν)/dσ), where ∆ν is the change in the Raman band position and σ is the stress, is invariant for cellulose fibers consisting either of cellulose I or cellulose II polymorphs.21,22 A value of the stress sensitivity of the Raman band shift has been found to be 4.7 cm-1/GPa,21,22 and this is the value used for the further analysis in the work presented here. The stress invariance indicates that the shift in the 1095-cm-1 Raman band is governed by this parameter, and such behavior is consistent with the structure being adequately described by the modified series aggregate model.21,22 In the tunicate sample, it is thought that this structure predominates, as there is little amorphous material, and there is no observed broadening. The stress invariance and the series model predict that the rate of Raman band shift with strain (d(∆ν)/ d) is proportional to the fiber modulus E via the equation E)
d(∆ν) dσ × d d(∆ν)
(1)
Equation 1 enabled the determination of the modulus of the two-dimensional random network of cellulose whiskers in the tunicin/epoxy composite from the rate of the Raman band shift with strain (d(∆ν)/d). By use of eq 1, the previously published21,22 value of 4.7 cm-1/GPa for the band shift rate with stress (d(∆ν)/dσ) and the magnitude of the Raman band sensitivity to strain (d(∆ν)/d) of 2.4 cm-1/% obtained in this work (cf. Figure 5), the tensile modulus of the tunicate cellulose sample was found to be 51.1 GPa. In fibers, the overall fiber strain is a result of the strain in the chain segments and the rotation strain, i.e., the rotation of the domains containing the chain segments.21,22 In low-modulus fibers, the crystallite domains are less oriented, and it has been proposed that crystallite reorientation would dominate the deformation of low-modulus cellulose fibers and that the chain stretching contributes less to the overall stress leading to smaller shifts in the position of Raman bands.22 Furthermore, Yeh and Young33 have shown for poly(p-phenylene benzobisoxazole) and Twaron fibers that lower modulus fibers display greater Raman band broadening, and the same trend has been observed for a group of regenerated cellulose fibers.23 Such behavior was suggested to correspond to a
Figure 6. Energy vs c spacing for a model cellulose I structure indicating the position of the equilibrium c spacing.
microstructure with a parallel-series arrangement of amorphous and crystalline domains, resulting in overstressed crystals and understressed amorphous regions.23,24 However, with a highly crystalline cellulose sample such as tunicin, the broadening effect is expected to be minimal, and this has been confirmed experimentally. Therefore, we conclude that we are observing relatively (to other cellulose I fibers studied) pure crystalline deformation. Additionally, since there was little relative reduction in intensity with the rotation of the sample, compared to a highly oriented cellulose I fiber, it is believed that it is also a two-dimensional random network of fibers. Application of analysis developed by Krenchel34 has previously enabled Eichhorn and Young20 to obtain a relationship between the tensile modulus Ef of a single reinforcing fiber and the tensile modulus Ec of a twodimensional random network of fibers as follows Ec ) η0Ef
(2)
where η0 is an efficiency factor developed by Krenchel.34 The application of this model assumes that the fibers are well compacted within the tunicate sheet. The efficiency factor η0 has a value of 9/8π for a random two-dimensional in-plane arrangement of fibers.34 Equation 2 was therefore used to estimate the value of a single tunicin cellulose whisker, Ef, by applying the value of 51.1 GPa for the tensile modulus, Ec, of the two-dimensional random network of cellulose whiskers and the efficiency factor, η0, with a value of 9/8π for this type of cellulose whisker organization in the tunicin/epoxy composites. Thus, the tensile modulus of a single cellulose whisker was found to be 143 GPa, which is in good agreement with a value of 137 GPa found for cellulose fibers of bleached ramie by measurement of cellulose lattice extension using X-ray diffraction.10 Furthermore, since this value is derived from a highly crystalline cellulose sample, we conclude that this value must be close to the true modulus of the material, and hence tunicin is a demonstrated analogue of a pure cellulose-I structure. Molecular Modeling. The energy of the cellulose I chain as a function of c spacing is shown in Figure 6. The minimum energy corresponds to the equilibrium (i.e., zero
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Table 1. Predicted Positions of Normal Modes with Associated Infrared Intensities mode no.
mode position (cm-1)
infrared intensity (arbitrary units)
62 63 64 65 66 67
1086.6 1091.9 1098.6 1099.3 1113.4 1117.2
0.2 11 19 0.4 15 0.9
Figure 8. Stress/strain curve for a model structure of cellulose I.
Figure 7. Mode wavenumber as a function of strain for a model cellulose I structure for Raman-active modes.
strain) c spacing, which is determined to be 10.46 Å for this structure. This is somewhat different from the value obtained by Woodcock and Sarko35 of 10.34 Å and a value2 for cellulose Iβ of 10.38 Å. However, these determinations were from crystalline structures using X-ray diffraction, and hence the values obtained in this study are likely to be different due to the lack of intermolecular hydrogen bonding. From the total number of 131 modes predicted for the cellulose I structure, a number of candidates for the 1095-cm-1 Raman band were chosen. These normal-mode positions, and intensities, are reported in Table 1. It is clear from the intensities of these modes that since modes 62, 65, and 67 have the lowest infrared intensity, and have positions closest to 1095 cm-1, then they are the most likely candidates for the Raman band that one would choose to represent the experimental data. Theoretical shifts in these bands with strain are shown in Figure 7. It is clear from these shifts that modes 62 and 67 are sensitive to deformation applied to the chain, whereas mode 65 is relatively insensitive and therefore less likely to be representative of the experimental shifts. The magnitude of the strain band shift rate of mode 67 (-2.5 cm-1/%) is in agreement with what has been seen with the experimental data (-2.4 cm-1/%). However, one must be cautious in making the conclusion that this proves that the model structure is in agreement with experiment, since there are a number of modes that are in close proximity, and therefore mode 62 is an equally viable candidate. The stress on the theoretical cellulose I chain has been found by first obtaining the first derivative of the third-order polynomial curve fit to the curve describing the variation of the energy with c spacing (cf. Figure 6) to obtain the force applied to the chain. This was then divided by an experi-
mental26 cross-sectional area of the chain (a × b) to obtain the stress, which is adjusted for this cell from the experimentally determined crystal unit cell.26 This computation yielded the stress/strain curve shown in Figure 8 and a value of the elastic modulus of the theoretical chain structure of 145 GPa. This is a value in the range of previously published values10-14 and slightly higher than the value of 143 GPa obtained in this study using Raman spectroscopy and Krenchel analysis. However, this discrepancy is marginal and shows that the modulus of tunicate cellulose whiskers is indeed very high and close to the theoretical limit for a cellulose structure. Conclusions It is clear that Raman spectroscopy is a powerful tool for the study of the microdeformation of highly crystalline cellulose samples. It has been demonstrated that by using the shift in a characteristic band within the Raman spectrum of a tunicate sample that the modulus of the material can be calculated. Furthermore, since there is no broadening of the Raman band upon deformation, it has been shown that this shift is related to direct chain stretching of cellulose and that relatively little amorphous/crystalline effects seen with semicrystalline cellulose fibers occur. This analysis has yielded a value of 143 GPa for the elastic modulus of a cellulose whisker, which agrees with a theoretical calculation of the modulus of a cellulose I chain of 145 GPa. The shift behavior of the cellulose composite shows that there is a possible breakdown of the interface, and this has been observed for other nanocomposite systems. The prediction of Raman band shifts has shown that it is possible to isolate one particular normal mode with a shift of the same order as that seen experimentally, which, although it gives an indication that this approach is valid, requires further work to prove this conclusively. Acknowledgment. One of the authors (S.J.E.) wishes to thank the EPSRC for funding this research under Grant No. GR/S44471/01. The authors also wish to thank Prof. R. J. Young for valuable discussions and Prof J. Y. Cavaille` for providing us with the tunicate cellulose sample.
Properties of Tunicate Cellulose Whiskers
References and Notes (1) Belton, P. S.; Tanner, S. F.; Cartier, N.; Chanzy, H. Macromolecules 1989, 22, 1615. (2) Nishiyama, N.; Langan, P.; Chanzy, H. J. Am. Chem. Soc. 2002, 124, 9074. (3) De Souza Lima, M. M.; Wong, J. T.; Paillet, M.; Borsali, R.; Pecora, R. 2003. Langmuir 2003, 19, 24. (4) Ruiz, M. M.; Cavaille`, J. Y.; Dufresne, A.; Graillat, C.; Ge´rard, J.F. Macromol. Symp. 2001, 169, 211. (5) Angle`s, M.; Dufresne, A. Macromolecules 2001, 34, 2921. (6) Harris, B. Engineering Composite Materials; IOM Communications Ltd: 1999. (7) Meyer, K. H.; Lotmar, W. HelV. Chim. Acta 1936, 19, 68. (8) Lyons, W. J. J. Appl. Phys. 1959, 30, 796. (9) Treloar, L. R. G. Polymer 1960, 1, 279. (10) Sakurada, I.; Nukushina, Y.; Ito, T. J. Polym. Sci. 1962, 57, 651. (11) Mann, J.; Roldan-Gonzalez, L. Polymer 1962, 3, 549. (12) Matsuo, M.; Sawatari, C.; Iwai, Y.; Ozaki, F. Macromolecules 1990, 23, 3266. (13) Kroon-Batenburg, L. M.; Kroon, J.; Northolt, M. G. Polym. Commun. 1986, 27, 290. (14) Tashiro, K.; Kobayashi, M. Polymer 1991, 32, 1516. (15) Mitra, V. K.; Risen, W. R.; Baughman, R. H. J. Chem. Phys. 1977, 66, 2731. (16) Batchelder, D. N.; Bloor, D. J. Polym. Sci., Part B: Polym. Phys. 1979, 17, 569. (17) Blackwell, J.; Vasko, P. D.; Koenig J. L. J. Appl. Phys. 1970, 41, 4375. (18) Atalla, R. H. Appl. Polym. Symp. 1976, 28, 659.
Biomacromolecules, Vol. 6, No. 2, 2005 1061 (19) Wiley: J. H., Atalla, R. H. Carbohydr. Res. 1987, 160, 113. (20) Eichhorn, S. J.; Young, R. J. Cellulose 2001, 8, 197. (21) Eichhorn, S. J.; Sirichaisit, J.; Young, R. J. J. Mater. Sci. 2001, 36, 3129. (22) Eichhorn, S. J.; Young, R. J.; Yeh, W. Y. Textile Res. J. 2001, 71, 121. (23) Eichhorn, S. J.; Young, R. J.; Davies, G. R. Biomacromolecules 2005, 6, 507. (24) Takayanagi, M.; Imada, K.; Kajiyama, T. J. Polym. Sci. C 1966, 15, 263. (25) Wise, L. E.; Murphy, M.; D’Addiecco, A. A. Paper Trade J. 1946, 122, 55. (26) Cael, J. J.; Gardner, K. H.; Koenig, J. L.; Blackwell, J. J. Chem. Phys. 1975, 62, 1145. (27) Bower, D. I.; Maddams, W. F. The Vibrational Spectroscopy of Polymers; Cambridge University Press: 1992. (28) Atalla, R. H.; Agarwal, U. P. Science 1984, 227, 636. (29) Mu¨ller, M.; Czihak, C.; Vogl, G.; Fratzl, P.; Schober, H.; Riekel, C. Macromolecules 1998, 31, 3953. (30) Mu¨ller, M.; Czihak, C.; Burghammer, M.; Riekel, C. J. Appl. Crystallogr. 2000, 33, 817 (31) Astley, O. M.; Donald, A. M. Biomacromolecules 2001, 2, 672. (32) Wood, J. R., Zhao, Q.; Wagner, H. D. Comp. Biochem. Physiol, A 2001, 32, 391. (33) Yeh, W.-Y.; Young, R. J. Polymer 1999, 40, 857. (34) Krenchel, H. Fibre Reinforcement; Akademisk Forlag: Copenhagen, 1964. (35) Woodcock, S.; Sarko, A. Macromolecules 1980, 13, 1183.
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